1. Field of the Invention
The present invention relates to a digital modem in which, in order to transmit a digital information signal such as a digitized high-definition television image signal by utilizing an idle channel in ground broadcasting of television, a carrier is modulated with the digital information signal and a received modulated carrier is demodulated to take out the digital information signal, and, more particularly, to this type of digital modem which can further reduce radio interference jamming in the same channel exerting from an adjacent broadcasting area.
2. Description of the Related Art
Conventionally, in ground broadcasting of television, the frequency is allotted every other channel to respective broadcasting areas to prevent adjacent channel jamming. It has hitherto been studied in U.S.A. to utilize an idle channel left in the frequency allotment affected every other channel so as to perform new television (ATV: Advanced Television) broadcasting such as high-definition television (HDTV) broadcasting without causing any trouble in reception of standard type television broadcasting. The ATV scheme is called simulcast scheme. Typically, idle channels for adjacent ground broadcasting areas are shifted from each other to avoid radio interference jamming in the same channel between the adjacent ground broadcasting areas. Therefore, when the simulcast utilizes an idle channel left in the frequency allotment for each broadcasting area, an adjacent ground broadcasting exerts radio interference jamming in the same channel on reception of the ATV broadcasting. This raises many difficulties in the development of ATV broadcasting.
Proposed in signal transmission of ATV broadcasting of simulcast type is the use of digital modulation which can advantageously permit signal transmission of the same quality as that of signal transmission in the conventional television ground broadcasting at a lower carrier to noise (C/N) ratio than that obtained with vestigial side-band amplitude modulation (VSB-AM) used in the television ground broadcasting, thus ensuring that reduction of reception trouble exerting on the existing VSB-AM type broadcasting can be achievable by making transmission power lower in ATV broadcasting than in VSB-AM type broadcasting. In the ATV broadcasting simulcast scheme using the conventionally common type of digital modulation, radio interference jamming in the same channel exerting from a newly built ATV station on the existing NTSC station is taken into account and resolved by reducing transmission power of the ATV station as described previously, but no countermeasure against radio interference jamming in the same channel exerting from the existing NTSC station on the newly, later-built ATV station has been taken into consideration. Accordingly, the desirable/undesirable (D/U) electric wave ratio of ATV broadcasting electric wave with reduced transmission power to NTSC broadcasting electric wave from adjacent broadcasting areas is degraded significantly, making it impossible to receive and demodulate the ATV broadcasting electric wave, and the reception trouble in ATV broadcasting is a significant problem to be solved in the course of development of ATV broadcasting.